A. Field of the Invention
The present invention relates to a method of preventing velocity data from causing an occurrence of potentially hazardously misleading attitude data. The velocity data may be used to aid in attitude leveling algorithms, for example, in an attitude/heading computer.
B. Problems in the Art
Airplanes use an attitude/heading computer (AHC) to provide the attitude/heading information of the plane. The AHC utilizes velocity information received from an external leveling aiding source to aid in its attitude leveling algorithms. The external leveling aiding source can include, for example, an air data computer (ADC), a GPS receiver, and others, such as are known in the art. The velocity information that an AHC receives for attitude leveling aiding can include, for example, true air speed, true altitude rate, indicated air speed, indicated altitude rate, ground speed, etc.
The AHC controls the attitude leveling function of the automatic flight control system in airplanes. The AHC also controls display of attitude information on primary flight displays within the cockpit. Airplanes will often have dual or even triple AHCs. In order to comply with Federal Aviation Administration (FAA) regulations, airplanes can be equipped with one ADC (or other leveling aiding source) for each AHC.
FAA Advisory Circular AC 25-11 classifies display of attitude in the cockpit as a critical function with the following specific guidelines concerning potential effects: (1) Total loss of all attitude display in the cockpit is classified as catastrophic and must be shown to have a probability of less than 1.0.times.10.sup.-9 /hour; (2) Loss of attitude display on both primary displays is major and it must be shown to have a probability of less than 1.0.times.10.sup.-5 /hour; (3) Hazardously misleading display of pitch or roll simultaneously on both primary flight displays is catastrophic and must be shown to have a probability of less than 1.0.times.10.sup.-9 /hour; and (4) Hazardously misleading display of pitch or roll on a single primary flight display is major and must be shown to have a probability of less than 1.0.times.10.sup.-5 /hour. The European Joint Aviation Authorities (JAA) have similar guidelines, with the exception that hazardously misleading display of pitch or roll on a single primary flight display is considered hazardous and must be shown to have a probability of less than 1.0.times.10.sup.-7 /hour.
In order to satisfy these regulations, a single ADC has previously served as an input for only one AHC. Primary flight displays were provided with pitch and roll comparators that compared pitch and roll data from each of two AHCs that provided primary display of pitch and roll data. The comparator warnings were displayed when pitch or roll disagreed by more than four degrees for longer than one second. Therefore, in order for the comparators to be "fooled," both AHC units had to exhibit failure modes that provided the same misleading pitch or roll information without warning. In installations where each AHC received data from a different ADC, both ADCs had to fail in an identical manner simultaneously to avoid comparator detection of a hazardous effect on pitch or roll data. The probability of a random hardware failure causing this in both ADC units is negligible.
While the method of providing a different ADC for each AHC satisfies safety regulations, this implementation also increases the overall cost of the airplane. In addition, providing separate ADCs for each AHC increases the amount of weight within the airplane. A method that could satisfy FAA and JAA regulations while eliminating the need for multiple ADCs would reduce the cost of an airplane by saving the expense of multiple ADCs. It would also reduce the expense of operating an airplane by eliminating the weight of the multiple ADCs. In addition, it would save the electrical energy that is currently required to power the multiple ADCs. There is a need in the art for a method and apparatus that complies with FAA regulations while allowing an ADC to be input into multiple AHCs.
Therefore, it is a primary object of the present invention to provide a method of preventing potentially hazardously misleading attitude data that solves problems and deficiencies in the art.
It is another object of the present invention to provide a method of preventing potentially hazardously misleading attitude data whereby a single ADC can be input into multiple AHCs while still satisfying FAA safety requirements.
It is a further object of the present invention to reduce the cost of airplanes, particularly by eliminating the extra expense associated with using a plurality of ADCs.
It is a further object of the present invention to reduce weight within an airplane.
It is a further object of the present invention to monitor leveling parameters to identify the possibility of hazardously misleading attitude data.
It is a further object of the present invention to provide a method which prevents an AHC from displaying hazardously misleading attitude information on a flight display.
Yet another object of the present invention is to provide a method which prevents an AHC from outputting hazardously misleading attitude data to an automatic flight control system.
It is a further object of the present invention that the threshold for the leveling parameters be of sufficient magnitude such that the occurrence of false detections is minimized when the air data is good.
These, as well as other objects and features of the present invention, will be apparent from the following detailed description and claims in conjunction with the accompanying drawings.